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THIN MIRROR WITH TRUSS BACKING AND MOUNTINGARRANGEMENT THEREFOR[0001] Aspects of embodiments of the present invention relate to a thin-sheet panelassembly, and more particularly to a thin-sheet panel assembly adapted for use in solarcollector applications. BACKGROUND [0002] Solar collection facilities utilize solar concentrators and/or photovoltaic panels forharnessing solar energy. Solar concentrator assemblies (SCAs) utilizing movable parabolictrough collectors present large reflective surface areas (apertures) for tracking the sun andfocusing the captured radiant energy on linear heat collection elements (HCEs) as a first stepin a thermomechanical conversion process for generating electrical power. A solar-troughsolar power generation facility typically includes many SCAs arranged in rows to capturegreat amounts of solar radiant energy.[0003] The reflective surfaces of troughs ofSCAs are usually hot-formed, thick-glassmirrors that ideally conform to a given geometry, notably surface curvature. Operatingefficiency of the solar plant is largely dependant on the ability of the mirrors to maintainsurface curvature accuracy so that the mirrors sharply focus reflected sunlight on the HCE.This requires very exacting manufacturing processes for mirror production and high rigidityof the mirrors themselves as mounted to their suppmis. Thus, the glass is typically formedthicker, often resulting in a mirror weight that rivals the weight of the suppm1ing structure.[0004] Traditionally, hot-formed, glass mirrors are used in various solar concentratingapplications, such as the Nevada Solar One solar power generating plant in Nevada. Theglass mirror material is hot-fonned to the mathematical shape (surface geometry) required toreflect and concentrate sunlight on an HCE. Such hot-formed glass is also known as saggedglass. Sagged glass is thick, heavy, costly to manufacture, costly to transport and install at afacility, and prone to breakage.[0005} The thick glass mirrors (or any other reflectors utilized) must retain theirmathematical shape in order to efficiently focus concentrated sunlight, which requires thehot-formed mirror glass to have a sufficient thickness to maintain the mirror shape (usuallyabout three to five millimeters). Problems with the hot-formed glass include that the thickerglass reduces the reflective efficiency of the mirror (more absorption and less reflection ofsunlight), has fabrication-related surface error limitations (slope error and edge effects), ismore costly due to the increased amount of glass material, and is heavier resulting inundesirable shipping and handling issues. Additionally, thick sheet material is difficult to-1-wo 2011/050329 PCT /US201 0/053856form in the complex shapes needed for solar power applications and may trap water in theinterface which corrodes the silvering.[0006] In some cases, thin-glass and thin-film have been bonded directly against a preformedsubstrate or aluminum plate having a desired curvature, or to a sandwich panel madewith the required surface geometry. Sandwich panels are usually comprised of two sheetmetal surfaces bonded to a cellular core such as a honeycomb configuration. Historicalproblems with thin-glass and thin-film bonded directly against a pre-formed curved substrateor plate, or in combination with honeycomb and other types of closed-cell panel construction,are high cost and their affinity for water intrusion when exposed to weather. Trapped waterattacks the preferably aluminum components and penetrates the mirror coatings, causing themirror silvering to corrode. SUMMARY [0007] Aspects of embodiments of the present invention are directed to a thin-sheet panelassembly. In embodiments of a thin-sheet panel assembly according to the present invention,the truss design of a backing material element is important to providing rigidity to the thinsheetpanel assembly. Embodiments of the thin-sheet panel assembly maintain a thin panel,such as a thin sheet of glass or other reflective material, in a rigid or substantially rigidconfiguration. Further, embodiments of the thin-sheet panel assembly of the presentinvention may be utilized in solar collector troughs of a solar concentrator assembly (SCA),for example.[0008] An aspect of embodiments of the thin-sheet panel assembly according to thepresent invention, as utilized in an SCA, for example, is lighter weight and less costlyreflective surfaces. For example, a thin mirror panel of a thin-sheet panel assembly,according to an embodiment of the present invention, may have about one fourth or less ofthe thickness and weight of sagged glass, and may be manufactured in a flat configurationwithout hot-forming (but may later be "cold formed" to a desired shape or curvature) and,therefore, may be manufactured at less cost. Another aspect of embodiments ofthe thin-sheetpanel assembly is the "cold" formability of the thin-glass or thin-film. Yet another aspect ofembodiments of the thin-sheet panel assembly is greater overall mirror rigidity and, therefore,improved accuracy as utilized in an SCA, for example. Still another aspect of embodimentsof the thin-sheet panel assembly is increased ease of installation. Still another aspect ofembodiments of the thin-sheet panel assembly is reduced transportation costs. Still anotheraspect of embodiments of the thin-sheet panel assembly is elimination or reduction ofsecondary breakage, such as may result from heavy wind forces applied against thick-glassstructures. Yet another aspect of embodiments of the thin-sheet panel assembly isinterchangeability with or use in combination with existing solar troughs or other devices.Still another aspect of embodiments of the thin-sheet panel assembly is improved reflectivewo 2011/050329 PCT /US201 0/053856performance. Yet another aspect of embodiments of the thin-sheet panel assembly is asubstantially open architecture for allowing moisture to drain or evaporate and therebyprevent or deter the accumulation of moisture between components ofthe assembly.[0009] According to one exemplary embodiment of the present invention, a substantiallyrigid thin-sheet panel assembly configured as a truss and having a non-rigid thin-sheetcomponent includes the thin-sheet component which has selected plan area and shape and is afirst chord of the truss, a backer having a plan shape and area substantially similar to the thinsheetcomponent and being a second chord of the truss, and plural riser elements of selectedheight and configuration each extending :from the backer to distal ends connected to a reversesurface of the thin-sheet component, the riser elements being diagonal elements ofthe trussand configured and disposed in an array which causes the assembly to have substantialrigidity in a selected direction in the thin-sheet component, and the thin-sheet panel assemblyfurther includes a bar coupled to the backer and extending between at least one pair ofadjacent riser elements of the plural riser elements.[0010] The riser elements may be of substantially uniform height :from the backer so thatthe thin-sheet component and the backer have essentially concentric curvature. An obversesurface of the thin-sheet component may have parabolically cylindrical concave curvature.The thin-sheet component may include a glass layer.[0011] The backer may be defined by a backer sheet, and the riser elements may bedefined by portions of the backer sheet that are bent away :from the backer sheet and haveintegral connections to the backer sheet. In one embodiment, the portions of the backer sheetthat define the riser elements have substantially hourglass-shaped configurations and theintegral com1ections to the backer sheet are at bases ofthe hourglass-shaped configurations.In one embodiment, the integral com1ections of the riser elements to the backer sheet arespaced along substantially parallel lines, and the riser elements are in substantially parallelplanes. In one embodiment, the bar extends between the substantially parallel planes.[0012] The riser elements at their distal ends may define connection tabs disposedlaterally of the heights of the risers. In one embodiment, the riser elements at their distal endsdefine respective pairs of connection tabs disposed laterally of the heights of the risers, theconnection tabs of each pair of connection tabs extending in opposite directions :from thedistal end of the riser element. The backer sheet may include aluminum.[0013] The connections of the distal ends of the riser elements to the reverse surface ofthe thin-sheet component may be bonded connections. The thin-sheet component mayinclude a substrate layer composed principally of a selected synthetic resin material. Thethin-sheet component may be reflective of electromagnetic radiation. In one embodiment, thethin-sheet component is reflective of solar radiation.[0014] In one embodiment, a first surface of the thin-sheet component opposite thereverse surface includes at least a portion of a curved reflective surface having a selectedwo 2011/050329 PCT /US201 0/053856curvature for directing and concentrating sunlight onto a receiver of a solar power generationfacility. The bar may be attachable to a trough frame supporting the curved reflectivesurface. The bar may have a hexagonal cross-sectional shape.[0015] According to another exemplary embodiment of the present invention, a supportstructure for supporting a thin panel to have a substantially rigid configuration includes abacker sheet, and a plurality of riser elements having heights extending in a first directionfrom proximal ends coupled to the backer sheet to distal ends and having lengths along rowsin substantially parallel riser planes, the distal ends of the riser elements being attachable tothe thin panel for supporting the thin panel and increasing rigidity of the thin panel in asecond direction substantially parallel to the riser planes, wherein the backer sheet isconfigured to be a first chord of a truss and the riser elements are configured to be diagonalelements of the truss, the thin panel being a second chord ofthe truss, and the supportstructure further includes a bar extending between at least one pair of adjacent riser elementsof the plurality of riser elements, the pair of adjacent riser elements being in a same one ofthe rows.[0016] The bar may extend between the rows in a third direction substantiallyperpendicular to the first and second directions. In one embodiment, the riser elements aredefined by portions of the backer sheet that are bent away from the backer sheet, haveintegral connections to the backer sheet, and have substantially hourglass-shapedconfigurations with the integral connections to the backer sheet being at bases of thehourglass-shaped configurations.[0017] According to another exemplary embodiment ofthe present invention, asubstantially rigid thin-sheet panel assembly having a non-rigid thin-sheet componentincludes the thin-sheet component which has selected plan area and shape, and a backerhaving a plan shape and area substantially similar to the thin-sheet component, the thin-sheetcomponent being attached to a first surface of the backer, wherein the backer includes abacker sheet including a plurality of ribs extending toward the thin-sheet component, and asubstrate including the first surface and a plurality of projections extending from a secondsurface opposite the first surface, each of the projections having a slot at a distal end andreceiving an end of a corresponding one ofthe ribs therein to form an interlocking joint.[0018] In one embodiment, a surface of the thin-sheet component includes at least aportion of a curved reflective surface having a selected curvature for directing andconcentrating sunlight onto a receiver of a solar power generation facility.[0019] Other features and advantages of embodiments of the present invention willbecome apparent from the following detailed description, taken in conjunction with theaccompanying drawings which illustrate, by way of example, features and aspects ofexemplary embodiments of the present invention.wo 2011/050329 PCT /US201 0/053856BRIEF DESCRIPTION OF THE DRAWINGS [0020] The above and other features, aspects, and advantages of the present inventionwill become better understood with regard to the following description, appended claims, andaccompanying drawings where:[0021] FIG. 1 is a front schematic view of a typical curved truss design;[0022] FIG. 2 is a perspective schematic view of a solar concentrator assembly;[0023] FIG. 3 is a fragmentary plan view of a backer for a thin miiTor according to anembodiment of the present invention, e.g., the backer being shown as formed to constitute acomponent of a miiTor panel assembly;[0024] FIG. 4 is an elevation view of the backer of FIG. 3 taken along line 4-4;[0025] FIG. 5 is an elevation view of the backer of FIG. 3 taken along line 5-5 showingthe backer as assembled into a miiTor panel assembly which is sho"\\on mounted to a supportfor the panel assembly;[0026] FIG. 6 is a sectional view of a thin-sheet panel assembly according to anotherembodiment of the present invention; DETAILED DESCRIPTION [0027] In the following detailed description, certain exemplary embodiments of thepresent invention are shown and described, by way of illustration. As those skilled in the artwould recognize, the described exemplary embodiments may be modified in various wayswithout departing from the spirit and scope of the present invention. Accordingly, thedrawings and description are to be regarded as illustrative in nature, rather than restrictive.Further, terms such as "upper," "lower," "top," "bottom," "upward," and "downward" are usedherein for the purpose of more clearly describing the location and/or orientation ofcomponents or features relative to others, as shown in the drawings, for example. However,the use of such terms is not intended or to be regarded as limiting the use of the invention toany particular position or orientation.[0028] Embodiments of the present invention are related to embodiments described inU.S. Provisional Patent Application No. 60/999,833 filed October 18, 2007; U.S. ProvisionalPatent Application No. 61/279,602 filed October 23, 2009; U.S. Patent ApplicationPublication No. US 2009/0101195 A1; and U.S. Patent 7,578,109 B2 issued August 25, 2009,the disclosure of each of which is incorporated herein by reference.[0029] With reference to FIG. 1, a typical curved truss structure 10 includes an upperchord 12, a lower chord 14, and diagonal elements 16, or lacing. The diagonal elements 16extend between and connect the upper and lower chords 12, 14 at various locations to providestrength and rigidity to the truss structure 10.[0030] With reference to FIG. 2, a solar concentrator assembly ("SCA") 20 includes arow of parabolic, cylindrically curved, or otherwise curved, troughs 22 for collecting radiantwo 2011/050329 PCT /US201 0/053856solar energy. The troughs 22 have reflective surfaces for reflecting and focusing the radiantenergy on a heat collection tube 25. Each ofthe troughs 22 is supported by a conespondingtrough frame 30, which maybe constructed oftubes, bars, extrusions, and/or any othersuitable structural members for supporting and maintaining the critical shape of each of thetroughs 22 and the reflective surfaces thereon. Each of the trough frames 30 may include twotorque plates 35, one on either side, for coupling to and supporting the trough frames 30 onsupporting pylons 40. The torque plates 35, may, for example, be coupled to the supportingpylons 40 at bearings 45.[0031] Some exemplary embodiments of the present invention are described herein in thecontext of a particular application and practical use of them, namely, a light reflective panelassembly 50 (see FIG. 5) useful to define a portion of a cylindrically curved, trough-likeminor, such as used with the trough 22 of the SCA 20 described above and shown in FIG. 2,as used in a solar thermal electric power generation facility. That is, the reflective surfaces ofthe troughs 22 ofthe SCA 20 described above and shown in FIG. 2 may be embodied as thethin-sheet panel assembly 50 shown in FIG. 5. The configuration of the thin-sheet panelassembly 50 provides rigidity to the thin minor and also maintains the reflective minorsurface in a shape having a desired curvature. As such, the reflective surfaces of the troughs22 will be configured to maintain a selected curvature for focusing solar radiation.[0032] In one exemplary embodiment, the reflective minor aspect of the panel assembly50 is a thin sheet minor 51. The thin sheet minor 51 can be a thin glass minor which isflexible due to its small thickness, or it can be a thin synthetic plastic film which includes alight reflective layer. In either instance, the thin sheet minor 51 is suitably bonded to astructural substrate 52 which is of sheet-like nature and which is sufficiently flexible that itcan be bent to the final curvature desired for the intended usage of the minor. The structuralsubstrate 52 together with the thin sheet mirror 51 function as an upper truss chord in thepanel assembly 10. In FIG. 5, according to an exemplary embodiment, the thin sheet mirror51 is a thin film mirror having a thickness of about 0.5 mm which is carried by a substrate(e.g., an aluminum sheet substrate) which has a smooth unfeatured surface to which themirror tilm is applied and secured. In another embodiment, the thin sheet minor may be athin glass mirror about 1.5 mm thick carried on a smooth surface metal (e.g., aluminum)substrate sheet. The substrate has a desired thickness defined so that it functions effectivelyas a top truss chord in the mini-truss minor panel assembly 50 shown in FIG. 5. In anotherembodiment, the structural substrate 52 may be omitted. In addition to substrate 52 and thethin minor 51 carried by it, the other principal component of panel assembly 50 is a backer60 which is depicted in FIGs. 3, 4, and 5. The backer 60 is defined to function as the lowerchord and the struts, or diagonals, (lacing) in the truss aspects of panel assembly 50.[0033] The backer 60 is defined by a backer sheet 61, such as an aluminum or stainlesssteel sheet, and in one exemplary embodiment is an aluminum sheet having a thickness of-6-wo 2011/050329 PCT /US201 0/053856about 0.020 inches. The backer 60 may be formed by use of a progressive die set, to defineplural risers 64 integrally connected at their bases to the backer sheet 61. That is, the risers64 function as the truss diagonals, while the remaining, or lower, portion of the backer sheet61 other than the risers 64 functions as the lower truss chord. As shown in FIG. 3, in oneembodiment, the risers 64 are integrally connected to and bent upward from the backer sheet61 to define openings 62 in the backer sheet 61. Alternatively, the risers 64 may beseparately formed and coupled to the backer sheet 61, such as via welding or an adhesive.The risers 64 are arranged in regularly spaced rows and columns across the length and width,respectively, ofthe backer sheet 61. The risers 64 are depicted in side elevations as seen inthe column direction in FIG. 4 and in front or face elevation as seen in the row direction inFIG. 5. In one embodiment, the riser base-to-base spacing in the column direction (FIG. 4)can be about 1.246 inch and in the row direction (FIG. 5) can be about 0.337 inch. The riserbase width can be about 1.1 inch. The riser height (FIG. 4) can be about 0.852 inch. Ofcourse, embodiments of the present invention are not limited by the above-describeddimensions and, in other embodiments, the risers 64 may have any other suitable dimensions.The riser top width preferably is substantially equal to the riser base width. The portion of ariser 64 between its base and top end is the body 65 of the riser 64. According to oneembodiment, in each column the bodies 65 ofthe several risers 64lie in a common plane, andthe planes of the several columns are parallel to each other. According to an exemplaryembodiment, each of those planes is perpendicular, at the riser 64 bases, to the plane of thebacker sheet 61 in which the risers 64 are defined.[0034] FIG. 5 shows that when seen in the row direction, each riser body 65 has an"hourglass" shape between its base and its top, in that the width of the riser 64 at its midheightis less than its base width. In one embodiment, the riser mid-height widths can beabout 0.5 inch. The side edges of each riser 64 from its mid-height to both its base and topends preferably are straight, as shown in FIG. 5.[0035] At its top end, each riser 64 is formed to define a pair of connection tabs or pads66 and 67 in a common plane perpendicular to the height of the riser 64. The pads 66 and 67extend in opposite directions from the riser 64. In one embodiment, pad 66 extends rearwardof the riser 64 (to the left as seen in FIG. 4) and pad 67 extends forward of the riser 64 (to theright as seen in FIG. 4). In one embodiment, the width of each pad in a directionperpendicular to the basic plane of its riser 64 can be about 0.15 inch. The top surfaces ofpads 66 and 67 enable the riser 64 to be affixed, as by gluing, to the reverse surface of mirrorsubstrate 62 in the course of the final fabrication of the panel assembly 50.[0036J The sloping side edges of the risers 64, produced by the "hourglass" shape of riserbodies 65, is desirable to reduce stress concentrations in the connections of the riser bases tothe backer sheet 61. Stress concentrations can arise in the backer at the riser bases for avariety of reasons, including the reason that the backer 60 is curved in the column directionwo 2011/050329 PCT /US201 0/053856of the backer 60 where, as noted above, the completed panel assembly 50 is to be used todefine a section of a parabolically curved cylindrical mirror in a solar trough solarconcentrator array. Such curvature of the panel assembly 50 is shown schematically in FIG.5 by radius of curvature 70 which may vary in length from place to place across the width ofthe panel assembly 50.[0037] In the formed-ready-for-assembly state ofthe backer 60 as shown in FIGs. 3 and4, and also in FIG. 5, the backer sheet 61 can define a plurality of ribs or corrugations 68which extend in the colunm direction ofthe backer sheet. The ribs 68 extend upwardly in thebacker sheet 61. Each rib 68 is located in the backer 60 just forwardly of the base ends of therisers 64 in a respective column of risers 64. Each rib 68 has a crest height above theadjacent surface ofthe backer sheet 61 which, in one embodiment, can be about 0.08 inches.The ribs 68 stiffen the backer 60 against deflection under compressive lower chord loadsapplied to the backer 60 in the direction ofthe ribs 68. Compressive chord load deflectionsof the upper chord of panel assembly 50 are resisted by the thickness of substrate sheet 52which can be thicker than the backer sheet 61 for that purpose. Also, in that state of thebacker 60, vertical stiffuess can be provided in the risers 64 as elongate dimples 69 in theriser material. The dimples 69, in an exemplary embodiment, have a vertical extent which isgreater than one-half the riser height, and they preferably are centered in the height of therisers 64. The ribs 68 and dimples 69 can be omitted if chord loads or panel assemblycompressive loads are expected to be low in use of a panel assembly.[0038] FIG. 5 shows how a fully fabricated panel assembly 50 can be mounted to amirror support frame, such as the trough frame 30 shown in FIG. 2 and/or a solar troughminor support frame ofthe V-top kind described in U.S. Patent 7,578,109 B2 issued August25, 2009, the disclosure of which is incorporated herein by reference. To efficiently mountand support the curved minor panel assembly 50, the mirror support frame preferablyincludes longitudinally extending mirror support tubes at spaced locations across the width ofthe frame. Those tubes preferably are defined by aluminum extrusions. In FIG. 5, a framemirror support tube, or frame stringer, is shown at 75. Stringer 75 includes a square tubeportion, part of which is shown in FIG. 5, from the top of which substantially coplanar lateralflanges 76 extend in opposite directions. Flanges 76 form structural features of the mirrorsupport frame which facilitate connection of the panel assembly 50 to the frame.[0039] As shown in FIG. 5, because ofthe "hourglass" shape of the bodies 65 oftherisers 64, a generally hexagonally shaped space is formed between adjacent risers 64 in eachcolumn of risers 64. In the row directions of panel assembly 50, those hexagonal spaces arelinearly aligned to create a ga11ery 77 of hexagonal cross-section shape between each adjacenttwo rows of risers 64. When the panel assembly 50 is placed properly adjacent frame stringer75 for connection of the panel assembly to the frame stringer 75, the adjacent surface oftheframe stringer 75 preferably subtends three galleries 77 (i.e. a central gallery and two side-8-wo 2011/050329 PCT /US201 0/053856galleries) with each side gallery lying adjacent a respective one of the stringer lateral flanges76. In one embodiment, a pressure bar 78 is present in each side gallery and extends alongthe length of its gallery. However, in other embodiments ofthe present invention, thenumber of pressure bars 78 on the panel assembly 50 and corresponding to each of the framestringers 75 may be varied depending on the application. The pressure bars 78 preferably areinserted into their respective receiving galleries before the backer 60 is placed in a formingmold (see U.S. Patent Application Publication No. US 2009/0101195 Al, the disclosure ofwhich is incorporated herein by reference, at paragraphs 0079-0085) for connection to themirror sheet 51 via its supporting substrate 52.[0040] Each pressure bar 78 has an exterior configuration which is similar to the crosssectionalshape of its hexagonal gallery 77 but with some clearance between the sides ofthepressure bar 78 and the side edges of the adjacent risers 64. That clearance enables the riser64 upper ends to move toward each other as the backer 60 deflects in the forming mold toconform to the mold curvature. The pressure bars 78 may be solid in cross-section, butpreferably are hollow to reduce their weight and cost. The pressure bars 78, in oneexemplary embodiment, are aluminum extrusions.[0041] The arrangement of pressure bars 78 between rows of risers 64 which is shown inFIG. 5 may be duplicated at each location in the panel assembly 50 which will be positionedadjacent a frame stringer 75. At an appropriate time, such as before placing the pressure bars78 in their hexagonal galleries or after removal of the panel assembly 50 from its formingmold, a tapped (internally threaded) hole 79 is formed through the backer sheet 61 and intothe adjacent portion of a pressure bar 78 at each of several spaced locations along the lengthof each pressure bar 78. For each hole 79, there is a corresponding hole 80, preferably notinternally threaded, fmmed through the pertinent stringer flange 76 at an appropriate location.The finished panel assembly 50, in one embodiment, is secured to the mirror support frameby use ofbolts 81 passed through flange holes 80 and threaded into the pressure bar holes 79to hold the panel assembly 50 securely on the support frame. If desired, a resilient washer, oring,or gasket element 83 may be placed between the bottom surface ofbacker sheet 61 andthe adjacent stringer flange 76 in association with each bolt 81. Also, if desired, a washer(not shown) can be positioned between each bolt head and the adjacent surface of flange 76.[0042] It is preferred, consistent with the material used to create the thin sheet mirror ofpanel assembly 50, that the components of a panel assembly 50 (i.e., its mirror substrate 52and backer 60) be defined by a material (or materials) which have thermal expansioncoefficients which match, as closely as possible, the thermal expansion coefficient ofthemirror mounting stringers, as well as the other components of the mirror support frame. Suchmatching ofthermal expansion coefficients results in the panel assembly being subject tominimal temperature related forces. Such forces can produce changes in the shape of thepanel assembly as it and its supporting structure experiences changes in temperature.-9-wo 2011/050329 PCT /US201 0/053856Changes in the shape ofthe panel assembly, notably the curvature of the mirror, can cause themirror to lose its ability to sharply focus reflected sunlight on a heat collector element locatedat the design focal point (or line) ofthe mirror.[00431 With reference to FIG. 6, a thin-sheet panel assembly 100 according to anotherembodiment of the present invention includes a backer 110, a substrate 120, and a thin sheet130 attached to an upper surface of the substrate 120. The thin sheet 130 may be a reflectivethin sheet, such as a thin-glass mirror or a thin-film mirror, similar to the thin sheet mirror 51described above. The backer 11 0 may be a polycarbonate sheet or any other suitablemateriaL Further, the backer 110 includes a plurality of ribs 115 extending from an uppersurface ofthe backer 110 toward the substrate 120. The ribs 115, in one embodiment, have asubstantially rectangular cross-sectional shape and extend in a longitudinal direction ofthepanel assembly 100. The substrate 120 maybe a polycarbonate sheet or any other suitablematerial. The substrate 120 also includes a plurality of projections 125 extending from alower surface of the substrate 120 toward the backer 110. The projections 125, in oneembodiment, have a substantially rectangular cross-sectional shape and extend in alongitudinal direction of the panel assembly 100. Further, the projections 125 are arranged atlocations corresponding to locations of respective ones of the ribs 115 for couplingcorresponding projections 125 and ribs 115 to each other. In one embodiment, as shown inFIG. 6, each ofthe projections 125 has a cup-shaped protrusion 126 at a lower end thereofand having a slot 128 formed therein for receiving an upper end of a corresponding one oftheribs 115. That is, the ribs 115 are coupled to corresponding ones ofthe projections 125 in theslots 128 to form interlocking longitudinal joints. According to one embodiment, the Iibs 115may be attached to the corresponding projections 125 with a solvent or other suitableadhesive or device.[0044] Similar to the panel assembly 50 described above and shown in FIG. 5, the thinsheetpanel assembly 100 may be a curved mirror panel assembly mounted to a mirrorsupport frame, such as the trough frame 30 shown in FIG. 2 and/or a solar trough mirrorsupport frame ofthe V-top kind described in U.S. Patent 7,578,109 B2 issued August 25,2009, the disclosure of which is incorporated herein by reference. To efficiently mount andsupport the curved mirror panel assembly 100, the mirror support frame preferably includeslongitudinally extending mirror support tubes, such as the stringers 75 described above, atspaced locations across the width of the frame. The thin-sheet panel assembly 100, in oneembodiment, is curved along a neutral plane 140 where, as noted above, the completed panelassembly 100 is to be used to define a section of a parabolically curved cylindrical mirror in asolar trough solar concentrator array. Such curvature of the panel assembly 100 is shownschematically in FIG. 6 by radius of curvature 145 which may vary in length from place toplace across the width of the panel assembly 100.-10-wo 2011/050329 PCT /US201 0/053856[0045] Although the drawings and accompanying description illustrate embodiments of athin-sheet panel assembly as applied to a solar collector trough, it will be apparent that thenovel aspects of the thin-sheet panel assemblies ofthe present invention may also be carriedout by utilizing alternative structures, sizes, shapes, and/or materials in other embodiments ofthe present invention. For example, in some embodiments of a thin-sheet panel assemblyaccording to the present invention, a thin panel, such as described above with respect to thethin-sheet panel assembly 50 as the thin sheet 51, may not be reflective, but rather, may beformed of an alternative material for purposes of decoration, strength, or otherwise.Embodiments of the thin-sheet panel assembly may, for example, be applied to any marketsector, such as, but not limited to, solar power generation, science, structural or decorativearchitecture, and industry.[0046] The preceding description has been presented with reference to variousembodiments of the invention. Persons skilled in the art and technology to which thisinvention pertains will appreciate that alterations and changes in the described structures andmethods of operation can be practiced without meaningfully departing from the principles,spirit, and scope ofthis invention.-11-